Vertebrate

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Vertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 57,739 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise fish (including lampreys, but traditionally excluding hagfish — as there is no evidence that it has a backbone — though this is now disputed), amphibians, reptiles, birds, and mammals (including humans).

Anatomy and morphology

Characteristics of the subphylum are a muscular system that mostly consists of paired masses, as well as a central nervous system which is partly located inside the backbone (if one is present). The defining characteristic of a vertebrate is considered the backbone or spinal cord, a brain case, and an internal skeleton, but the latter do not hold true for lampreys, and the former is arguably present in some other chordates. Rather, all vertebrates are most easily distinguished from all other chordates by having an unequivocal head, that is, sensory organs - especially eyes are concentrated at the fore end of the body and there is pronounced cephalization. Compare the lancelets which have a mouth but not a well-developed head, and have light-sensitive areas along their entire back.

Evolutionary history

Vertebrates originated about 530 million years ago during the Cambrian explosion, which is part of the Cambrian period. The earliest known vertebrate is Myllokunmingia. According to recent molecular analysis Myxini (hagfish) also belong to Vertebrates. Others consider them a sister group of Vertebrates in the common taxon of Craniata.

Taxonomy and classification

Classification after Janvier (1981, 1997), Shu et al. (2003), and Benton (2004).

• Subphylum Vertebrata
• (Unranked group) Hyperoartia (lampreys)
• Class †Conodonta
• Subclass †Pteraspidomorphi
• Class †Thelodonti
• Class †Anaspida
• Class †Galeaspida
• Class †Pituriaspida
• Class †Osteostraci
• Infraphylum Gnathostomata (jawed vertebrates)

* Class †Placodermi (Paleozoic armoured forms)

* Class Chondrichthyes (cartilaginous fish)

* Class †Acanthodii (Paleozoic "spiny sharks")

* Superclass Osteichthyes (bony fish)

* Class Actinopterygii (ray-finned fish)

* Class Sarcopterygii (lobe-finned fish)

* Subclass Coelacanthimorpha (coelacanths)

* Subclass Dipnoi (lungfish)

* Subclass Tetrapodomorpha (ancestral to tetrapods)

* Superclass Tetrapoda (four-limbed vertebrates)

* Class Amphibia (amphibians)

* Series Amniota (amniotic embryo)

* Class Sauropsida (reptiles and birds)

* Class Aves (birds)

* Class Synapsida (mammal-like reptiles)

* Class Mammalia (mammals)

Etymology

The word vertebrate derives from Latin vertebrātus (Pliny), meaning having joints. It is closely related to the word vertebra, which refers to any of the bones or segments of the spinal column.

References

Bibliography

See also

• Invertebrate
• Marine vertebrates

External links

• Tree of Life
• Tunicates and not cephalochordates are the closest living relatives of vertebrates

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Last updated on Wednesday March 12, 2008 at 16:27:14 PDT (GMT -0700)
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